Using known rotational moulding techniques, moulded articles can be produced having a solid or unfoamed "skin" or envelope and a cellular interior, the latter having a density in the region of 0.15 g/cm.sup.3. The composite structure is rigid and of lower overall density than many common woods, and can be used for the manufacture of chemically resistant pallets, boxes, containers and similar goods.
Typical examples of plastics materials used in the production of hollow articles having cellular interiors produced by a rotational moulding technique are polyethylene as the wall, envelope or skin material of the article, and polyurethane as the foam. Polyethylene has useful chemical resistance and toughness and polyurethane foam is rigid and provides the desired degree of stiffness.
There is disclosed in CA 983226 (Du Pont) a method of rotational moulding of polyolefin articles having a foamed inner layer and a substantially solid skin. Molded articles are made in a one-step rotational moulding process using a mixture of powdered non-foamable ethylene polymer, which forms the outer skin, together with a foamable ethylene polymer in pellet form, which forms the foamed inner layer.
In the process of the Canadian patent, the particle size difference as between the foamable pellets and the powdered non-foamable polymer enables a separation of these materials to be achieved during the rotational moulding process so that the moulded product has the necessary solid outer envelope and associated foamed lining.
We have conducted tests of the process described in the Canadian patent and these show clearly that in practice only very thin and uneven skins or envelopes are achieved when using the conditions described in the Canadian patent. These very thin skins have occasional thick spots, but the inadequacy of the skin thickness is such that the process is not really practical, and this is maybe the reason why this one-shot process has never been commercially exploited. Further, the foamed material does not completely fill the moulded product and instead only forms a relatively thin lining on the interior surface of the skin, which limits the effectiveness of the material. Furthermore, we have found that if more of the foamable polymer is added in an attempt to fill the mould, the powder and the pellets do not separate properly, resulting in even poorer skin formation.
We have found that in a one-shot rotational moulding process it is not sufficient merely to provide the two main components of the mould charge with differing particle sizes. While such difference in particle size can achieve a degree of separation of the components to enable formation of the impermeable outer envelope and the foamed material within it, in the absence of other contributory factors as defined below, the process as described in the Canadian patent does not achieve an acceptable wall or envelope thickness.